This invention relates generally to video discs and video disc players, and, more particularly, to techniques for detecting and measuring eccentricity in such discs and players.
Video discs typically record video information signals in a sequence of substantially circular and concentric recording tracks surrounding a center hole. The recorded signal is normally recovered by a player apparatus having a spindle for engaging the center hole and rotating the disc in a prescribed fashion, and an information recovery device for scanning the rotating disc while moving radially from track to track. The information recovery device typically includes means for directing a reading beam of light at the rotating disc, which reflects the beam in accordance with the recorded information, and a photodetector for detecting the modulated intensity of the reflected beam.
Problems in recovering the recorded signal can sometimes arise if the center hole in the disc is not precisely centered (i.e., is eccentric) with respect to the concentric recording tracks. When this occurs, the radius of each recording track changes cyclically as the disc rotates. A similar effect arises if the spindle itself is eccentric with respect to its axis of rotation. To overcome these eccentricity problems, the player normally includes a tracking servo for controllably moving the point of impingement of the reading beam on the disc to follow a selected track. This technique is ordinarily effective, however, only if the radial excursion of the track is less than a prescribed amount, e.g., 100 track spacings.
During the manufacture of video discs, it is desirable to measure the degree of eccentricity of each disc. One prior technique has been to rotate the disc on a player apparatus while disabling the tracking servo such that the reading beam scans the disc at a fixed radius. Counting the number of tracks that cross beneath the beam during each revolution then provides a measure of eccentricity. An alternate technique is to monitor the relative timing of a periodic signal included in the playback signal, as is done for example in a conventional time base error correction system, and to measure the magnitude of any deviation of that periodic signal relative to a fixed local reference signal.
The two techniques described above have proven generally effective in providing accurate measurements of video disc eccentricity. However, there is a need for an even simpler technique, which does not require the inclusion of special circuitry in the player apparatus. The present invention fulfills this need.